The invention relates to a process for fining oxide melts, particularly glass melts.
In relation to melts, fining is understood to mean the removal of gas bubbles from the melt. In order to achieve maximum freedom from unwanted gases and bubbles, thorough mixing and degassing of the molten mixture, for example, the glass, is required.
The behavior of gases and bubbles in a glass melt and the removal thereof is described, for example, in H. Jebsen-Marwedel and R. Bruckner, "Glastechnische Fabrikationsfehler," Third Edition, Springer-Verlag, p. 195 ff., as well as in Uhlman and Kneidl, eds., "Glass Science Technology," Vol. 2, Chapter I, Michael Cable, pp. 16-24.
Generally speaking, in principle, two different fining processes are known, which differ essentially by the nature and manner of fining gas generation.
In mechanical fining, water vapour, oxygen, nitrogen or simply air are forced in through openings in the bottom of the melting unit. In this so-called bubbling process, the melt then becomes thinner as a result of a further increase in temperature, and the gas bubbles can rise more easily to the surface. This stage of the process is also known as "bubble removal". In the bubbling process, freedom from unwanted gases is often improved by agitators. As the bubble size of the fining gases forced in is generally too large, however, and the gas bubbles rise too quickly, the extremely high degrees of freedom from unwanted gases required for melting optical glasses, for example, are achieved only with great difficulty, even by agitator support.
Chemical fining processes are used most frequently. Their principle lies in the fact that compounds that decompose and dissociate gases, or compounds that are volatile at relatively high temperatures, or compounds that liberate gases in an equilibrium reaction at relatively high temperatures are added to the melt.
Sodium sulphate (Glauber's salt), for example, belongs to the first group of compounds, which cleaves sulphur dioxide and oxygen at about 1,200.degree. C. and is preferred as a cheap raw material for fining mass-produced glasses.
The compounds that are volatile at high temperatures because of their vapour pressure and are effective as a result include, inter alia, NaCl or certain fluorides.
Finally, the last group of substances includes the so-called redox fining agents such as, for example, arsenic oxide, antimony oxide or cerium oxide etc. In this by far the most frequently used process in practice, polyvalent ions which may occur in at least two oxidation states are used as redox fining agents, which ions are in a temperature-dependent equilibrium with one another, a gas, mostly oxygen, being liberated at high temperatures.
The redox equilibrium of the substance dissolved in the melt can be represented by the equation (I), taking arsenic oxide as an example EQU As.sub.2 O.sub.5 =As.sub.2 O.sub.3 +O.sub.2 .uparw. (I)
The equilibrium constant K for (I) may be formulated as in equation (II): ##EQU1##
In this equation, .sup..alpha. As.sub.2 O.sub.3 and .sup..alpha. As.sub.2 O.sub.5 mean the activities of arsenic trioxide and arsenic pentoxide, and .sup.P O.sub.2 means the fugacity of the oxygen.
The equilibrium constant K is highly temperature-dependent, and a defined oxygen fugacity .sup.P O.sub.2 can be adjusted by means of the temperature and the activities of the oxidic arsenic compounds.
Both in mechanical and chemical fining, essentially three fining effects may be distinguished:
1) a primary fining effect due to the spontaneous formation or introduction of gas bubbles, preferably oxygen bubbles, during the use of redox fining agents, whereby the unwanted gases dissolved in the melt, for example CO.sub.2, N.sub.2, H.sub.2 O, NO, NO.sub.2 and others, diffuse into the gas bubbles. The gas bubbles thereby become inflated, and the inflated gas bubbles rise upwards more quickly, eventually leaving the melt; PA1 2) a secondary fining effect in which the reverse process to the one described under 1) takes place, namely the diffusion of gases, for example oxygen, out of the redox equilibrium into unwanted gas bubbles present, so that said bubbles become inflated and receive an increased uplift and PA1 3) a so-called resorption effect in which inflated bubbles of, for example, oxygen, produced according to 1) or 2) and still present in the melt when the temperature is reduced dissolve, for example, in the case of the redox equilibrium (I) as a result of the equilibrium being shifted to the side of the starting product.
A common feature of all chemical fining processes is that chemicals which are harmful to the environment, but at least not environmentally acceptable, are added to the melts. In addition, volatilisation fining agents, fluorides, may be mentioned for example, or arsenic or antimony oxides which act as redox fining agents. Already, certain substances can be used only on a very restricted scale now (fluorides, arsenic oxide) or in the near future (antimony oxide), and there are plans to prohibit their use altogether. Alternative redox fining agents, for example, cerium oxides, are relatively expensive substitutes.
Apart from mechanical and chemical fining, there have also been attempts to fine oxide melts by electrochemical means.
A process for fining oxide glass melts is known, for example, from U.S. Pat. No. 3,775,081, in which the fining gas is generated in situ in the oxide melt in an electrochemical reaction. To this end, small quantities of molten metal are required on the bottom of a melting vessel in order to generate hydrogen gas in an electrochemical reaction from water vapour which is present in the glass melt or has diffused therein, which gas is claimed to serve as a fining gas for fining the melt.
The process described in U.S. Pat. No. 3,775,081 is, however, associated in various ways with considerable disadvantages. The process is restricted to gas-heated melting tanks, since water vapour can be made available in such quantity as is sufficient to generate the hydrogen fining gas only in tanks heated with gas or oil, but not in electrically heated melting tanks. The presence of a source of water vapour in the melt to be fined--whether the water vapour results from burning fuel or, another possibility, whether it is introduced--is extremely disadvantageous in this connection because the melt must, in principle, be kept free from water vapour for effective fining in order to avoid the "reboil effect" of the melt.
In addition, with the process according to U.S. Pat. No. 3,775,081 there is the risk that concentrations of metal ions of polyvalent metals other than the desired concentrations will be obtained in the glass produced. This may be the consequence of a varying partial pressure of water vapour becoming lower during the course of fining, which leads to a shift in the redox equilibrium of polyvalent ions, such as the reduction of Fe.sup.2+ to Fe.sup.3+. Moreover, the requirement that the metal for reducing the water vapour in the process according to U.S. Pat. No. 3,775,081 must be present in the molten state in order to achieve a sufficient reactivity of the metal restricts the process--depending on the glass melt--to tin, lead, antimony or nickel as metal. The converse conclusion is, therefore, that not every glass melt can be fined in this way. Moreover, a whole series of glasses is excluded a priori from the use of the fining process of U.S. Pat. No. 3,775,081, however, because there is a risk that the glass components will be reduced by the molten metal. The greatest disadvantage, however, is likely to lie in the use of hydrogen as fining gas. The gaseous hydrogen may react immediately with oxygen in an explosive manner on leaving the melt.
Apart from electrochemical fining, in which gas bubbles are generated in the glass melt for the refining thereof, it is also known, for example, from GB-A-1,128,561, that the formation of gas bubbles produced by electrochemical reactions can be prevented in glass melts that have already been fined. In this connection, GB-A-1,128,561 advocates keeping a glass melt in an electrically conducting tank under a non-oxidising atmosphere in order to avoid the renewed occurrence of gas bubbles after fining.
Although GB-A-1,128,561 describes a principle according to which the development of gas bubbles in the melt appears to be explainable, the conclusions with regard to fining are incorrect or wholly absent. In GB-A-1,128,561, a so-called platinum/glass (T.sub.1)-glass (T.sub.2)/platinum thermocell is described, the short circuit of which leads to oxygen bubble formation in the "reboil effect". Contrary to the stated principle, however, oxygen formation may occur not only at the higher temperature electrode but also at the lower temperature electrode (Baucke, Mucke in Journal of Non-Crystalline Solids 84 (1986), page 174 ff). Moreover, the stated principle has another error. This consists in that the development of oxygen at the higher temperature electrode does not yet lead to bubble formation that can be used for fining, only to oxygen formation. In order to form bubbles from oxygen that can be used for fining, the higher temperature must lie in the vicinity of the reboil temperature.
Moreover, no indication can be derived from the entire prior art concerning electrochemical fining processes as to how the kinetics of the fining reaction are to be controlled, or how the thermodynamics of the fining reaction can be mastered and used in a purposeful manner. In other words, no method is shown as to how the number of bubbles and the size of the fining gas bubbles could be adjusted.